Guiding rangelands into an uncertain future

The oncoming stream of biophysical and social changes facing the rangelands calls for an innovative mix of modifying existing uses and pursuing new ones to help transition into a social-ecological system more in tune with its new environment. In the face of rising uncertainty, trying to find some particular, optimal combination of management and policy to suit some assumed future environment can’t work. A transformational change is needed. This does not mean getting to one new, particular kind of system. It has to be an ongoing process of exploring, learning and keeping options open, along the lines of the developing field of guided self-organisation.

Energy Extraction Through Flapping Foils

We demonstrate experimentally that flapping foils within an oncoming stream can efficiently extract energy from the flow, thus offering an attractive, alternative way for energy production. The greatest promise for flapping foils is to use them in unsteady and turbulent flow, where their own unsteady motion can be controlled to maximize energy extraction. The foils in this study perform a sinusoidal linear motion (sway, or heave) in combination with a sinusoidal angular motion (yaw or pitch); the effect of three principal parameters is studied systematically, yaw amplitude, the Strouhal number, and the phase angle between sway and yaw. The foils are made of aluminum, in the shape of NACA 0012 airfoils, using three different aspect ratios, 4.1, 5.9, and 7.9; they were tested at Reynolds numbers around 14,000. Efficiencies of up to 52 ± 3% are achieved with simple sinusoidal motions, thus demonstrating that foils can efficiently extract energy from unsteady flows.

Cavitation Nuclei Population and Event Rates

To model the processes of cavitation inception, noise and damage, it is necessary to generate a model of the cavitation event rate which can then be coupled with the consequences of the individual events to produce a complete synthesis of the phenomenon. In this paper we describe recent efforts to connect the observed event rates to the measured distributions of cavitation nuclei in the oncoming stream. Comparisons are made between the observed event rates and event rates calculated from measured nuclei distributions using an algorithm which includes the dynamics of the nuclei motion and growth. Various complications are explored including the effect of the boundary layer, the relative motion between the nucleus and the liquid, the observable bubble size effect, and the effect of bubble growth on neighboring nuclei. All of these are seen to have important influences on the event rate, and therefore, on cavitation inception and other macroscopic consequences. We demonstrate that it is possible to predict the correct order of magnitude of the event rate when an attempt is made to model the important flow complications.

On the Organization of Flow and Heat Transfer in the Near Wake of a Circular Cylinder in Steady and Pulsed Flow

Skin friction, pressure, and heat transfer gages are employed to monitor the flow and heat transfer field along the periphery of a circular cylinder in steady and pulsed flow at Reynolds numbers, Re = 23,000 to 50,000. Averaged distributions, RMS, and power spectra of all measurements are displayed. Special attention is directed at the organization of the near wake, as detected by the three types of surface gages. The response of the wake to pulsing of the oncoming stream is also examined. It is found that when the wake is locked on the driving frequency, the basic character of the flow is not changed, but the organized motion stands out more clearly. Moreover, the signals become cleaner and background noise in the spectra is reduced. Skin friction and heat transfer gages are shown to respond to local variations of the corresponding quantities, whereas pressure gages respond to global characteristics of the flow.

Some Effects of Upstream Turbulence on the Unsteady Lift Forces Imposed on Prismatic Two Dimensional Bodies

Measurements of fluctuating lift are reported from four fixed rectangular prisms and a fixed circular cylinder, for various intensities of turbulence in the oncoming stream. Variations of unsteady lift with intensity of small scale turbulence are similar to present (and previous) measured variations of mean drag or base pressure. Reasons are suggested for these trends. Observations are also reported of the maximum (resonant) RMS amplitude of two cylinders, one square and the other circular in cross section, free to move in the cross stream direction. Small scale free stream turbulence provided by the wake of a small upstream rod increased the resonant amplitude of motion in both cases, the greater increase (and subsequent decrease) occurring for the square section. Two possible explanations for these observations are advanced.

Perturbation Solutions of Transient Heat Transfer to a Porous Medium from a Fluid Stream

Asymptotic behavior of the two-phase, thermal response of a porous medium, with a fluid throughflow, to a sudden change of the oncoming stream temperature is examined via four cases of perturbation solutions: (1) massive blowing, fast interphase (thermal) link, small conductivity, and small (thermal) interaction at the entrance, (2) moderate blowing, and fast interphase link, (3) massive blowing, moderate interphase link, and small interaction at the entrance, and (4) small conductivity, as well as weak interphase link. Schumann’s classical, two-phase, and non-diffusive model is shown to be just the outer limit of Case (1). The “single-phase model” is derived. Associated boundary layers are displayed. Certain numerical trend reported in the literature is also explained.